Origins: au pair, the buck stops here
The origin of "au pair," discussed in this Wordwatch column by Merriam-Webster, is interesting. It started out in English an adjectival or adverbial phrase, closer to its original meaning in French: "She is seeking a position au pair."
"Au pair" would be literally translated as "at par," but the column says it's closer in meaning to "on equal terms." The "equal" here has more to do with reciprocal arrangement than with an au pair being treated as an equal part of the family; the au pair would agree to, say, care for children in exchange for room and board. Or maybe daughters in two families would switch places to teach the other their language.
Just as in England, "au pair" in the United States usually refers to a young foreign woman. While adopting "au pair," we've also picked up on the British term "nanny" to use for a usually older woman, whether foreign or American, who has similar duties but whose job specifically involves child care. An au pair isn't necessarily in charge of children, though in the United States she frequently is.The column also discusses the origin (Harry S. Truman) of "the buck stops here," a phrase I'm rather fond of, though I never use it.